The present invention is generally concerned with mobile radiocommunication systems.
The present invention is more particularly concerned with power control techniques used in such systems to improve performances (in terms of quality of service, of capacity, . . . etc.).
The present invention is in particular applicable to mobile radiocommunicotion systems of CDMA (xe2x80x9cCode Division Multiple Accessxe2x80x9d) type. In particular, the present invention is applicable to UMTS (xe2x80x9cUniversal Mobile Telecommunication Systemxe2x80x9d).
As is known, CDMA systems use two types of power control techniques, a so-called open-loop power control technique, and a so-called closed loop power control technique (also called hereinafter CLPC). These power control techniques may be recalled for example for the uplink transmission direction, i.e. from MS (xe2x80x9cMobile Stationxe2x80x9d) to BTS (xe2x80x9cBase Transceiver Stationxe2x80x9d). In the open-loop power control, a MS transmit power is controlled based on the power received by this MS from a BTS. In the CLPC, a MS transmit power is controlled based on the transmission quality of the link between this MS and a BTS, as estimated at this BTS.
The transmission quality of a link between a MS and a BTS depends on the ratio of the received signal power and the interference power, also called SIR-Signal-to-Interference Ratio). When the SIR of a MS is low, or equivalently when the powers of the other MSs are much higher than its power, its performances dramatically decrease. The CLPC algorithm enables to keep the SIR of each user as constant as possible.
The principle of the CLPC algorithm is that the BTS periodically estimates the SIR of the received signal from each MS, and compares this estimated SIR to a target SIR (SIRtarget). If the estimated SIR is lower than the target SIR, the BTS sends a power control command to the MS, for the MS to increase its transmit power. Otherwise, the BTS sends a power control command t0 the MS, for the MS to decrease its transmit power. The target SIR is chosen by the BTS as a function of the required quality of service. The target SIR may itself be adjusted according to a quality of service target value, by a slower loop also called outer loop as opposed to the preceding one called inner loop.
Transmission rate in such systems may be made variable, for various reasons, such as for example:
use of variable rate services, such as for example data packet services,
transmission in compressed mode, whereby informations are momentarily transmitted at a rate higher than necessary in order to leave some periods idle for performing other tasks (such as for example radio measurements for handover preparation reasons, in particular inter-frequency handover),
optimisation of system performances (a reduction in the transmission rate, wherever possible, enabling to improve quality and/or capacity), . . . etc.
As is known, in CDMA systems the transmission rate may be made variable by spreading the informations to be transmitted with a variable spreading factor. The lower the spreading factor, the higher the transmission rate.
Besides, and as also known, the higher the transmission rate, the higher the required transmit power (for a same quality of service).
However, in a system including a CLPC algorithm of the above-recalled type, it may take a relatively long time to reach the new required power each time there is a change in the transmission rate, for various reasons including in particular the time it takes for the outer loop to adjust the target SIR accordingly, or the fact that the transmit power is adjusted in a stepwise manner by the inner loop.
The CLPC algorithm may also be adapted, to make this time as short as possible. To this end, EP 0 886 389 teaches to change the transmit power in an inverse proportion to the variation of the spreading factor.
An object of the present invention is to improve such an adaptation of a CLPC algorithm.
In particular, an object of the present invention is not only to reduce the time it takes for the CLPC to reach the new required power, but also to set the new required transmit power to an optimised value.
Optimising the value of the new required transmit power is highly beneficial in such systems; indeed, if the required power is set to a value higher than necessary, there is a needless contribution to interference level in the system, and therefore a needless degradation of the performances of the system; on the other hand if the required power is set to a value lower than necessary, the performances of the on-going communication will be degraded.
An object of the present invention is therefore a method for improving performances of a mobile radiocommunication system using a power control algorithm for controlling a transmit power according to a transmission quality target value, said method comprising, upon the occurrence of a significant change in the required transmit power, bypassing said power control algorithm by changing the transmit power according to a corresponding change in the required transmission quality target value.
According to another object of this invention, said significant change in the required transmit power includes a change in the transmission rate.
According to another object of this invention, said corresponding change in the required transmission quality target value has a predetermined value.
According to another object of this invention, said predetermined valve may be regularly updated.
According to another object of the present invention, said transmission quality is represented by a signal-to-interference ratio.
According to another object of this invention, said mobile radiocommunication system is of CDMA type.
According to another object of this invention, said power control is performed in the uplink transmission direction of said mobile radiocommunication system.
According to another object of this invention, said power control is performed in the downlink transmission direction of said mobile radiocommunication system.
Another object of the present invention is a mobile station comprising, for performing such a method in said uplink transmission direction:
means for bypassing said power control algorithm, by changing the transmit power according to a corresponding change in the required transmission quality target value, upon the occurrence of a significant change in the required transmit power.
According to another object of this invention, such means include a look-up table, containing predetermined values of corresponding changes in the required transmission quality target value, corresponding to different significant changes in the required transmit power.
Another object of the present invention is a mobile radiocommunication network entity comprising, for performing such a method in said uplink transmission direction:
means for correspondingly changing the required transmission quality target value, upon the occurrence of a significant change in the required transmit power.
According to another object of this invention, such means include a look-up table, containing predetermined values of corresponding changes in the required transmission quality target value, corresponding to different significant changes in the required transmit power.
Another object of the present invention is a mobile radiocommunication network entity comprising, for performing such a method in said downlink transmission direction:
means for bypassing said power control algorithm, by changing the transmit power according to a corresponding change in the required transmission quality target value, upon the occurrence of a significant change in the required transmit power.
According to another object of this invention, such means include a look-up table, containing predetermined values of corresponding changes in the required transmission quality target value, corresponding to different significant changes in the required transmit power.
Another object of the present invention is a mobile station comprising, for performing such a method in said downlink transmission direction:
means for correspondingly changing the required transmission quality target value, upon the occurrence of a significant change in the required transmit power.
According to another object of this invention, such means include a look-up table, containing predetermined values of corresponding changes in the required transmission quality target value, corresponding to different significant changes in the required transmit power.